1. Field of the Invention
The present invention relates to a chromatic dispersion generating apparatus capable of generating the required chromatic dispersion to perform the chromatic dispersion compensation of optical signals, the chromatic dispersion tolerance measurement of an optical components or the like, and in particular, to a chromatic dispersion generating apparatus configured by utilizing an optical component provided with a function of demultiplexing an input light according to wavelengths.
2. Description of the Related Art
In an optical fiber communication system, the chromatic dispersion compensation is performed using, for example, a dispersion compensation fiber (DFC) having a characteristic opposite to a chromatic dispersion characteristic of a laid single mode fiber (SMF) or the like. Since a chromatic dispersion value of the DCF is adjusted according to the length thereof, there are many cases where the total fiber length necessary for compensating for the required chromatic dispersion is previously calculated, to prepare a plurality of DCF reels of different lengths. Therefore, at a system installation site, the chromatic dispersion value can be set based on only the combination of the prepared DCF reels. Accordingly, sometimes, it is hard to realize the optimum chromatic dispersion compensation.
To such a DCF, as one of chromatic dispersion compensators capable of freely setting the chromatic dispersion value, there has been proposed a chromatic dispersion compensator configured utilizing a so-called virtually imaged phased array (VIPA) for demultiplexing an input light into a plurality of optical beams that can be distinguished spatially according to wavelengths (refer to Japanese Unexamined Patent Publication No. 9-43057 and Japanese National Publication No. 2000-511655).
FIG. 13 is a perspective view showing a configuration example of a conventional VIPA-type chromatic dispersion compensator. Further, FIG. 14 is a top view of the configuration example of FIG. 13.
As shown in each figure, in the conventional VIPA-type chromatic dispersion compensator, a light emitted from one end of an optical fiber 130 via an optical circulator 120 is converted into a parallel light by a collimate lens 140 and, then, condensed on one segment by a line focusing lens 150 and passes through a radiation window 116 of a VIPA plate 110 to be incident between opposed parallel planes. The incident light on the VIPA plate 110 is multiple-reflected repeatedly, for example, between a reflective multilayer film 112 formed on one plane of the VIPA plate 110 and having the reflectance lower than 100% and a reflective multilayer film 114 formed on the other plane and having the reflectance of substantially 100%. At this time, every time the incident light is reflected on the surface of the reflective multilayer film 112, a few % of the light is transmitted through the surface to be emitted outside the VIPA plate 110.
The lights transmitted through the VIPA plate 110 interfere mutually and form a plurality of optical beams, traveling directions of which are different from each other, according to wavelengths. As a result, if each of the optical beams is condensed to one point by a convergent lens 160, each condensed position moves on a straight line according to variation of the wavelengths. By disposing, for example, a three-dimensional mirror 170 on the straight line, the lights that have been emitted from the VIPA plate 110 and condensed by the convergent lens 160 are reflected at different positions on the three-dimensional mirror 170 according to respective wavelengths to be returned to the VIPA plate 110. Since the lights reflected on the three-dimensional mirror 170 are propagated through an optical path through which the lights have been propagated previously, in an opposite direction, different wavelength components are propagated for different distances and, therefore, the chromatic dispersion of the input light is compensated.
In consideration of a model as shown in FIG. 15, for example, behavior of the light that is multiple-reflected by the VIPA plate 110 as described above is similar to that in an Echelon grating that is well-known as a step-wise diffraction grating. Therefore, the VIPA plate 110 can be considered as a virtual diffraction grating. Further, in consideration of an interference condition in the VIPA plate 110, as shown on the right side in FIG. 15, the emitted light interferes under a condition in which, with an optical axis thereof as a reference, a shorter wavelength is above the optical axis and a longer wavelength is below the optical axis. Therefore, among a plurality of optical signals of respective wavelengths, optical signals on the shorter wavelength side are output above the optical axis and optical signals on the longer wavelength side are output below the optical axis. Such a conventional VIPA-type chromatic dispersion compensator has advantages in that the chromatic dispersion can be compensated over a wide range and also the wavelength (transmitted wavelength) of a light to be compensated can be varied.
In the conventional VIPA-type chromatic dispersion compensators as described above, even if optical characteristics, such as the chromatic dispersion value, the transmitted wavelength and the like, are the same, internal setting values (for example, the position of the three-dimensional mirror, the temperature of the VIPA plate and the like) are different, due to individual differences of components of VIPA optical system. Therefore, the applicant of the present invention has proposed a technology for obtaining to store data relating to a chromatic dispersion value and a transmission wavelength characteristic in each wavelength corresponding to the position of the three-dimensional mirror, the temperature of the VIPA plate and the like, before starting an operation of the chromatic dispersion compensation, and then, at the operation time, reading the data corresponding to setting conditions such as the chromatic dispersion value and the like, out of the stored data, to control the position of the three-dimensional mirror and the temperature of the VIPA plate (refer to Japanese Unexamined Patent Publication No. 2003-311083).
However, in the above described prior invention, basically, the data relating to all of the wavelengths and the chromatic dispersion values which are assumed to be used at the operation time should be previously acquired. Therefore, in order to realize a general-purpose VIPA-type chromatic dispersion compensator capable of coping with more wavelengths and more chromatic dispersion values, there is a caused problem in that a long time is required for acquiring the data.
Further, the above described conventional VIPA-type chromatic dispersion compensator has been reviewed, as another usage thereof, to be utilized for the chromatic dispersion tolerance measurement of devices or modules used in the optical communication. In this usage, the configuration shown in FIG. 13 is utilized as a chromatic dispersion emulator operating at arbitrary wavelengths. In the above described usage for the chromatic dispersion compensation, since optical signals corresponding to a wavelength grid in conformity with the ITU-T or the like are basically objects of the chromatic dispersion compensation, the above data may be acquired corresponding to respective wavelengths on the grid, as the stored data. On the contrary, in the usage as the chromatic dispersion emulator, since devices and modules to be used on the wavelengths outside the wavelength grid in conformity with the ITU-T or the like are objects of the chromatic dispersion tolerance measurement, the above data corresponding to the arbitrary wavelength should be previously acquired to be stored. Therefore, in order to realize the desired measurement accuracy, there is a caused problem in that it takes a significantly long time required for acquiring the data, and also a memory of large capacity is necessary.